An ultrathin, rapidly fabricated, flexible giant magnetoresistive electronic skin

In recent years, there has been a significant increase in the prevalence of electronic wearables, among which flexible magnetoelectronic skin has emerged as a key component. This technology is part of the rapidly progressing field of flexible wearable electronics, which has facilitated a new human perceptual development known as the magnetic sense. However, the magnetoelectronic skin is limited due to its low sensitivity and substantial field limitations as a wearable electronic device for sensing minor magnetic fields. Additionally, achieving efficient and non-destructive delamination in flexible magnetic sensors remains a significant challenge, hindering their development. In this study, we demonstrate a novel magnetoelectronic touchless interactive device that utilizes a flexible giant magnetoresistive sensor array. The flexible magnetic sensor array was developed through an electrochemical delamination process, and the resultant ultra-thin flexible electronic system possessed both ultra-thin and non-destructive characteristics. The flexible magnetic sensor is capable of achieving a bending angle of up to 90 degrees, maintaining its performance integrity even after multiple repetitive bending cycles. Our study also provides demonstrations of non-contact interaction and pressure sensing. This research is anticipated to significantly contribute to the advancement of high-performance flexible magnetic sensors and catalyze the development of more sophisticated magnetic electronic skins.

The manufacturing process of GMR devices is as follows: 1. Spin-coating of PI Film: -A 4-inch silicon wafer was prepared using spin coating to deposit a polyimide (PI) film, with a spinning speed set at 1200 rpm.

Patterning GMR Devices:
-Photolithography techniques were employed to pattern the GMR devices.The dimensions of each GMR device were 100 µm x 1000 µm.Seven of these devices were serially connected in a folded configuration.Ion beam etching (Advance, LKJ-1A-150) was utilized to etch the GMR strips.4. Deposition of Dielectric Layers: -Chemical vapor deposition (CVD) methods were used to deposit dielectric layers of silicon dioxide (SiO2) and silicon nitride (SiNx), each with a thickness of 150 nm.These layers served as electrical isolation.Photolithography techniques were also employed to create openings for electrode connections.

Electrode Definition:
-Photolithography techniques were used to define the electrode shapes.Electrodes were deposited in the following order: 15 nm of tantalum (Ta) followed by 100 nm of gold (Au).6. Annealing: -All GMR devices were annealed in a vacuum at 180°C for 1 hour under a bias magnetic field of 10,000 Oe.During the annealing process, the GMR stripes were oriented perpendicular to the direction of the applied exchange bias field.This annealing step was carried out in a Vacuum Magnetic Annealing Furnace (Model F800-35/EM7, East Changing Technologies, China). 7. Electrochemical Delamination: -Finally, an electrochemical delamination method was employed to detach the flexible GMR devices.This comprehensive process outlines the fabrication of the GMR devices, incorporating various deposition techniques and lithography methods to achieve the desired structure and properties.The figure depicts the surface profile of a 3-micrometer scan along the edge of the PI film.There is a protrusion at the edge of the PI foil caused by cutting, which can be disregarded.The remaining portion can be used to calculate the thickness, with the dashed line corresponding to a thickness of 0.97 µm.The manufacturing process of magnetic skin：Magnetic skin is made of elastic matrix and permanent magnetic powder.The elastic matrix materials is Ecoflex(Smooth-on, 00-50), the magnetic powder can be NdFeB.Figure S1 (a) illustrates the fabrication process: first, the composite material is prepared by mixing the elastic matrix and permanent magnet powder.And then the 3D printing mold is prepared with the required shape and size.The mixture is cast into the mold, and the casting blade is used for planarization.After curing, the magnetic skin is magnetized along the out of plane direction, and finally the composite material is released from the mold to make the magnetic skin.The magnetic skin is magnetized using 2 T external magnetic field for 1 hour.Figure 1 (b) shows the scanning electron microscope (SEM) of the magnetic skin, in which the black particles are permanent magnet powder and the light part is elastic matrix.The materials of the elastic matrix and the magnetic powder, as well as the mixing ratio, will be discussed in Figure 1 (c) to illustrate the influence on the properties of the magnetic skin.The diagram illustrates the readout process for an array of resistors.Initially, rows to be read are selected using a multiplexer (RS2251XTSS16) and subjected to a working voltage of 1.25V provided by a voltage regulator (REF3012AIDBZR).Subsequently, the circuit interfaces with a GMR sensor array and an operational amplifier (TLV2374Q), with a reference resistor of 1.5kΩ to match the resistance of the GMR device.The operational amplifier is connected to a 16-bit analogto-digital converter (AD7606).Finally, data is transmitted to a computer via serial communication using a microcontroller (STM32F407VET6) to display changes in the magnetic field above the GMR sensor array.A pressure sensing unit was constructed using Comsol and subjected to finite element simulation.The unit is composed of a 1.8 mm magnetic layer and a 2.2 mm non-magnetic layer, as same in the manuscript.The area where force is applied is set as a circle with a radius of 1 mm.The presence of the non-magnetic layer helps to expand the range of force detection.When a force of 0.4N is applied, the stress change of the pressure sensing unit is as shown in Figure S7(a), and deformation can be observed in the area where the force is applied due to the soft characteristics of two layers.Based on this deformation, a magnetic field simulation was further added to calculate the magnetic field distribution on the bottom surface under this deformation.Figures 6-S7(a, b, c) show the magnetic field distribution in the x, y, and z directions, respectively.

Figure S2 .
Figure S2.The step profiler photographs of the PI foil.

Figure S3 :
Figure S3: Atomic force microscope images of the Si substrate before and after spincoating with polyimide are presented, providing a comparison of the roughness before and after the spin-coating process.The average roughness measured across six points on the wafer was found to be 0.163 nm (Figure S3(a)), after spin-coating with polyimide, the average roughness increased to 0.574 nm (Figure S3(b)).

Figure S4 .
Figure S4.(a) Fabrication process of the magnetic skin; (b) T Scanning electron microscope characterization of the magnetic skin; (c) Taking Ecoflex as elastic matrix, the characterization of the modulus of elasticity and remanent magnetizations versus different content of the magnetic powder in the magnetic skin.

Figure S5 .
Figure S5.The signal processing circuit of GMR devices.

Figure S7 .
Figure S7.Finite element simulation of piezomagnetic electronic skin pressure sensing: (a) Stress simulation after applying force; (b, c, d) Magnetic field distribution in the x, y, z direction after applying force.